Energy changes in reactions

Exothermic and endothermic

• Exothermic: releases heat; temperature rises; ΔH is negative (combustion, neutralisation).
• Endothermic: absorbs heat; temperature falls; ΔH is positive (dissolving ammonium nitrate, photosynthesis).

Bond energies

Breaking bonds absorbs energy; forming bonds releases energy.

ΔH = energy to break bonds - energy released forming bonds

Calculating ΔH from experiments

1. Heat change = mcΔT (c of water = 4.2 J g⁻¹ K⁻¹).

2. Moles reacting = given data.

3. ΔH = - heat change ÷ moles (negative for a rise in temperature).

Example: Neutralising 0.05 mol of acid raised 100 g of solution by 6.5 K.

Heat = 100 × 4.2 × 6.5 = 2,730 J. ΔH = -2,730 ÷ 0.05 = -54.6 kJ/mol.

Types of enthalpy

• Combustion: 1 mole burns completely in oxygen.
• Neutralisation: forms 1 mole of water (about -57 kJ/mol for strong acid and strong base).
• Solution: 1 mole dissolves.

Hess's law

The total enthalpy change is the same whatever route is taken.

Fuels

A good fuel: high heat value, easy to ignite, burns cleanly, cheap and available.